Many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are typically sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by creasing and sealing laminated strip packaging material.
The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas and light-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package ultimately contacting the food product.
Packages of this sort are normally produced on fully automated packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is conveniently removed from the surfaces of the packaging material, e.g. evaporated by heating. The web thus sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed vertically with the food product.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections.
More precisely, the tube is sealed longitudinally and transversally to its own axis. From this sealing operation, pillow packs are obtained, which have a longitudinal seal and respective top and bottom transversal seals.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are then filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name Tetra Rex (registered trademark).
Each pillow pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin projecting from respective sealing bands; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by respective trapezoidal walls.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing line.
Packaging machines of the above type are known, on which the pillow packs are turned into folded packages by automatic folding units.
Folding units are known, for example from EP-B-0887261 in the name of the same Applicant, which substantially comprise:                a chain conveyor for feeding packs continuously along a forming path from a supply station to an output station;        a number of folding devices arranged in fixed positions relative to the forming path and cooperating with packs to perform relative folding operations thereon;        a heating device acting on respective triangular flaps of each pack to be folded, to melt and seal them onto respective walls of the pack; and        a pressing device cooperating with each pack to hold the triangular flaps in the respective folded positions as these portions cool.        
The step of transferring and feeding pillow packs to the inlet station of the folding unit is critical for proper operation of the folding unit.
In practice, pillow packs are typically formed and sealed with their longitudinal axis arranged vertically. Subsequently, the newly formed pillow packs are let slide along a curved-profile chute so as to be brought from the vertical position to the horizontal position, in which they are received by a feeding conveyor, arranged immediately downstream of the chute and which drives the pillow packs to the inlet station of the folding unit.
A need is felt in the industry for a feeding unit which is capable of ensuring very smooth motion conditions for the pillow packs being fed to the receiving station of a folding unit. In particular, it is highly desirable that a unit for feeding pillow packs to a folding unit subject the pillow packs to dynamic conditions as consistently homogeneous and smooth as possible, while ensuring that the very transfer of the pillow packs from the conveyor to the receiving station may always be properly timed. At the same time, it is always preferable that pillow packs be subjected to as few yanks and stresses as possible, as they advance towards the folding unit.